This invention relates to optoelectric devices, their methods of use, and, in particular, methods of using photonic crystals.
Photonic crystals are structures having a periodic variation in dielectric constant. A one-dimensional example of a photonic crystal is an alternating stack of dielectric layers, which includes the well-known "quarter-wave stack" dielectric mirror. Light of the proper wavelength at normal incidence to such a mirror is completely reflected by the mirror. One description of the basis for this complete reflection is destructive interference between multiply-scattered waves of light inside the stack. An alternative description is that the solutions for propagating electromagnetic (EM) modes within the periodic stack give rise to a photonic "band" structure, in which propagating EM modes having frequencies within a "photonic bandgap" cannot exist within the periodic stack. EM waves having these frequencies are therefore completely reflected by the stack when incident on the stack. This band theory for EM waves is analogous to the band theory developed for electrons in crystals.
In general, the position and width of a photonic bandgap will depend on the propagation direction of the propagating EM wave and its polarization. In multi-dimensional photonic crystals (i.e., materials having a periodic dielectric constant along more than one dimension), a photonic bandgap can exist over a larger range of propagation directions. For example, a three-dimensional (3D) photonic crystal, such as a solid structure formed from dielectric spheres at the sites of a diamond lattice, can have a "complete photonic bandgap". Within such a photonic crystal, no propagating EM modes having frequencies within the complete photonic bandgap can propagate, regardless of propagation direction and polarization.
By fabricating photonic crystals having specific periodicities, the properties of the photonic bandgap can be tailored to specific applications. For example, the central wavelength of a photonic bandgap is approximately equal to the periodicity of the photonic crystal and the width of the photonic bandgap is proportional to the differences in dielectric constant within the photonic crystals. For a general reference, see: J. D. Joannopoulos et al., Photonic Crystals, (Princeton University Press, Princeton, 1995), the contents of which are incorporated herein by reference.
Photonic crystals can also include defects with respect to their periodicity that support a localized electromagnetic mode having a frequency within a photonic bandgap. For example, in a three-dimensional photonic crystal formed from dielectric spheres at the sites of a diamond lattice, the absence of a sphere produces a defect. In the immediate vicinity of the absent sphere, the photonic crystal is no longer periodic, and a localized electromagnetic mode having a frequency within the photonic bandgap can exist. This defect mode cannot propagate away from the absent void, it is localized in the vicinity of the defect. Thus, the introduction of a defect into the photonic crystal creates a resonant cavity, i.e., a region of the crystal that confines EM radiation having a specific frequency within the region. Resonant cavities are used in optical devices such as lasers and light-emitting diode displays, as well as components for microwave and millimeter wave applications, such as filters and power generators. Brommer et al. describes fabricating such devices by introducing defects into photonic crystals in U.S. Pat. Nos. 5,187,461, 5,389,943 and 5,471,180, the contents of which are incorporated herein by references. In these applications, separate dielectric waveguides channel the confined EM radiation into, and away from, the defect region.
In other applications, a photonic crystal includes a series of defects that combine to form a waveguide within the photonic crystal. The series of defects support EM modes having frequencies within the photonic bandgap that can propagate along the series of defects but are otherwise confined to regions of the photonic crystal in the vicinity of these defects. Defect-based waveguides in photonic crystals can include sharp turns since the photonic bandgap can prevent propagation of the EM radiation away from the waveguide for at least some, if not all, propagation directions. Waveguides based on photonic crystals and incorporated into optoelectronic integrated circuits are described by Meade et al. in U.S. Pat. No. 5,526,449, the contents of which are incorporated herein by reference.